Shark: Skeletal System, Muscular System, Digestive System, Respiratory System, and, Circulatory System
Skeletal System
The Shark’s skeleton is made of cartilage. The skeleton can be split into two main groups, as it can in other vertebrates: the axial skeleton, which consists of the skull, gills, and vertebral column, and the appendicular skeleton, which consists of the pectoral and pelvic girdles and their corresponding fins. The neurocranium and splanchnocranium make comprise the chondrocranium. The dorsal region of the skull, known as the neurocranium, guards the brain and some sensory organs. It is made up of several regions. The frontmost part is called the rostrum. The precerebral cavity is a hollow structure located in the dorsal region of the rostrum. The precerebral fenestrae are located at the caudal end of a ridge on the ventral surface known as the rostral carina. The nasal capsules’ remains can be seen on either side of the rostrum. These are open to the outside through the external nares in an intact specimen and include the olfactory epithelium. Causally, we can observe the broad optic orbits. Each orbit’s supraorbital crest, which shields the eyeball, is the shelf-like dorsal surface. Numerous foramina or openings can be seen inside each orbit. The optic nerve travels through the optic foramen, which links the eye with the brain. Trigeminal and facial nerves enter through the trigeminofacial foramen, which is located anterior to the optic foramen. The upper jaw and the posterior orbital process articulate in the eye. There are several superficial ophthalmic foramina located above the optic orbit. The otic capsules, which house the inner ears, are caudal to each orbit. The epiphyseal foramen, a small aperture through which the pineal body, or epiphysis, projects from the brain, is located on the medial dorsal surface of the skull between the anterior edges of the supraorbital crests. The endolymphatic fossa, a significant depression, is located on the dorsal surface as well. The endolymphatic foramina are located at its anterior end, and the perilymphatic foramina are located at its caudal end. The tissues covering the endolymphatic fossa and the inner ears are connected by these foramina and their associated ducts. The two paired occipital condyles that articulate with the vertebral column are located near the caudal end of the skull. The spinal cord exits the braincase and enters the vertebral column through a wide aperture called the foramen magnum that lies between them. The medial vagal foramen, through which the vagus nerve goes, and the lateral glossopharyngeal foramen, through which the glossopharyngeal nerve flows, are two foramina that are situated on either side of each occipital condyle.
The visceral cranium, sometimes called the splanchnocranium, is made up of seven visceral arches. The jaws are formed by the first two visceral arches. The jaws are formed by the mandibular arch. It undergoes modification to become the paired Meckel’s (mandibular) cartilages, which make up the lower jaw, and the paired palatoquadrate cartilages, which make up the upper jaw. The teeth are held in and supported by the jaws. The hyoid arch, which has been divided into two parts, is the second visceral arch. The paired, dorsal hyomandibular cartilages support the jaws by being joined to the otic area of the skull. The unpaired basihyal cartilage and the paired ceratohyal cartilage make up the ventral section of the hyoid arch. These serve as the mouth’s floor and support the tongue. The branchial arches make up the last five visceral arches. Three to six branchial arches have gill rays that support the gill tissues in addition to supporting the gills themselves. The seventh arch has no gill rays and is not connected to a gill. Only the tail vertebrae, also known as caudal vertebrae, and the trunk vertebrae, also known as body vertebrae, make up the dogfish’s vertebral column. A ventral centrum and a dorsal neural arch make up a normal trunk vertebra. The centrum is the vertebra’s primary structure. On both ends, it is deeply concave, or amphicoelous. The notochord is located in a tiny canal that passes through the centrum. From each side of the neural arch, short transverse processes stretch laterally. These processes are joined by thin ribs. The spinal cord is enclosed and safeguarded by the neural arch. A pair of triangular neural processes that unite dorsally to form the neural spine make up the neural arch. The ventral root of a spinal nerve goes through a foramen that is located close to the base of each neural process. An intercalary plate, which completes the spinal cord’s protective housing, is located between each neural process. The dorsal root of a spinal nerve travels through a tiny foramen found in each intercalary plate. The caudal, or tail, vertebrae resemble the trunk vertebrae in structure. The transverse processes of a tail vertebra are absent. Underneath the centrum, there is a noticeable ventral hemal arch, nevertheless. Two blood arteries, the caudal artery, and the caudal vein are located within this arch.
The pectoral and pelvic girdles, as well as their corresponding fins, make up the appendicular skeleton. The ventral side of the trunk is encircled by the pectoral girdle. A midventral coracoid bar with paired scapular and suprascapular cartilages make up this structure. At the glenoid surface, each pectoral fin articulates with the pectoral girdle. Three sets of basal cartilages make up the fin itself. The radial cartilages, which extend from these basal cartilages, provide the fin considerable stiffness. The radial cartilages project fibrous, dermal fin rays, or ceratotrichia, which support the distal part of the fin. More straightforward than the pectoral girdle is the pelvic girdle. Just anterior to the cloaca, a single ischiopubic bar can be found in the ventral abdominal wall. There is a little iliac process at each end. The acetabulum is where the pelvic fins and pelvic girdle join forces. Each fin has two basal cartilages, radial cartilages that radiate from them, and dermal fin rays that support the distal part of the fin. In males and females, the pelvic fins are different. Two highly modified radial cartilages in the male called claspers help transport sperm to the female during copulation.
Muscular System
Sharks have a fairly simple muscle structure. The majority of the musculature is made up of huge, striated, segmented, V-shaped muscle groups termed myotomes, which are easily visible in a skinned shark. The exceptions to this rule are specific muscles that govern the jaws and particular fins. A connective tissue-based myoseptum divides individual myotomes from one another. Additionally, a horizontal septum divides them into dorsal and ventral groupings.
Digestive System
The lower jaw to the aperture of the cloaca, as well as transverse incisions cranial to the pelvic girdle and caudal to the pectoral girdle, are necessary to access the shark’s internal organs. The coelom, or bodily cavity, is seen by folding back portions of the body wall. The digestive and reproductive systems are housed in the pleuroperitoneal cavity, also known as the abdominal cavity, while the heart is enclosed in the pericardial cavity. The two cavities are divided by the transverse septum.
Numerous membranes and mesenteries cover and support the organs within the coelom. The visceral peritoneum covers the surface of the organs, while the parietal peritoneum lines the inside of the body wall. The dorsal mesentery, a double-walled membrane that holds the organs in the abdominal cavity, connects the visceral and parietal peritoneums dorsally. The oral cavity is located behind the mouth, where the digestive system starts. There have several pointed teeth on the jaws. The shark has homodont teeth, which are essentially uniform in shape. The teeth, which are altered placoid scales, are regularly replaced. The fixed tongue rests on the floor of the mouth and is supported by two cartilages. The pharynx, a cavity that connects the mouth cavity with the oesophagus, is located posterior to the mouth. Six pairs of holes on the lateral walls are seen when the pharynx is opened. The first pair are the spiracles’ internal entrances. The internal gill slits are the next five pairs. The gill rakers, which stop food particles from entering the gill chambers, are visible in the gill slits.
The three-lobed liver is the largest and most noticeable organ in the abdominal cavity. The cavity is largely filled by the massive, extended right and left lobes. It is substantially smaller in the medial lobe. The liver, the largest organ in the body, serves a variety of purposes.
It does five different things to help the body function properly: it makes bile, which helps with fat digestion; it controls blood sugar levels by storing excess energy as oil and releasing it when more is needed; it helps with nitrogenous waste metabolism and disposal; it breaks down toxins and old red blood cells, and it makes a factor that helps with blood clotting.
The gallbladder, a little sac that may seem green due to the presence of bile, is located on the right side of the medial lobe of the liver. The common bile duct allows the gallbladder to release stored bile into the small intestine. The liver is joined to the ventral body wall by the falciform ligament. The remainder of the digestive system can be seen after the liver is removed. At its junction with the oesophagus, a narrow tube lined with many cone-shaped papillae that travels to the stomach, the posterior end of the pharynx is restricted.
The oesophagus attaches to the stomach’s cardiac area. The stomach lining contains several glands that create the acids and enzymes needed to break down food. The shark swallows huge chunks of food rather than chewing it, and the meal may stay in the stomach for a considerable time. The rugae, or numerous longitudinal ridges, are seen when the stomach is opened. The rugae are creases in the stomach wall that provide gastric glands with more surface area to work with and enable the stomach to grow when it is full of food. The pyloric area is located in the stomach’s caudal portion. The muscular pyloric valve at the end of this pathway controls how food enters the gut. The small intestine is thought to start at the pyloric valve, even though there is not a definite separation between the big and small intestines as there is in tetrapods. The short duodenum is the first part of the gut. The ileum is the longer and bigger section. The spiral valve, a special shark adaption that significantly improves the surface area accessible for nutrition absorption without increasing the overall length of the intestine, is visible when the ileum is opened. The short large intestine, also known as the colon, joins the short rectum, which goes to the cloaca. The digestive, excretory, and reproductive systems all share a chamber in the cloaca. The typical opening for these systems is the cloacal opening.
The whitish, glandular pancreas, which is located near the bend where the stomach and intestine meet, is an auxiliary digestive organ. The pancreas has two lobes: a long, narrow dorsal lobe and a circular, flattened ventral lobe. A brief isthmus connects the two lobes. The pancreas performs both exocrine and endocrine activities. It generates and secretes pancreatic enzymes necessary for digesting because it is an exocrine gland. These pass through the pancreatic duct and into the small intestine. It generates insulin as an endocrine gland to control blood sugar. The spleen is typically examined at this time due to its location in the abdomen, even though it is a component of the circulatory system rather than the digestive system. The stomach is only weakly connected to this black, triangular organ. The spleen controls the amount of circulating blood by producing white blood cells, removing damaged red blood cells, and storing blood.
Respiratory System
Like in all fish, the gills and the supporting tissues and blood vessels that they are connected to make up the respiratory system. The internal pharynx gill slits are located there. These slits over the gills allow water to enter, and the external gill slits let water exit. Oxygen diffuses into the circulation and carbon dioxide diffuses out of the blood as water travels over the highly vascularized gills.
A visceral arch supports each gill. Efferent and afferent arteries carry blood to and from the gills. Each gill is supplied with deoxygenated blood by a single afferent artery, and oxygenated blood is removed from the gill by two efferent arteries, one on either side of the afferent artery. Each visceral arch’s gill rays extend into the interbranchial septum, a cartilaginous support system. The visceral arch projects gill rakers into the throat. Each septum bears gill lamellae on each side, except the most anterior hyoid arches. Lamellae are only present on the posterior side of the hyoid arch. A hemibranch is a single set of lamellae, but a holobranch is a whole gill made up of two hemibranchs connected by an interbranchial septum.
Circulatory System
The heart, blood arteries, and blood are the components of the circulatory system. In the pericardial cavity, the heart is located. The pericardium of the parietal lobe lines the cavity. The visceral pericardium, another protective membrane, surrounds the heart itself. Every fish has four chambers in its heart. The atrium and the ventricle, however, are the two main chambers. Only deoxygenated blood is pumped around by the fish’s heart. Through the sinus venosus, which has a thin wall, blood enters the heart. The paired common cardinal veins enter the sinus venosus from opposite sides.
Blood from the liver is brought into the sinus venosus just lateral to the midline by the paired hepatic veins. Blood flows from the sinus venosus into the lengthy atrium with a thin wall. Blood flows into the tiny yet muscular ventricle from the atrium. The conus arteriosus, a large, muscular tube that transports blood to the ventral aorta, is used by the ventricle to propel blood to all areas of the body. To receive the blood that is being expelled from the ventricles, the conus arteriosus expands. It then contracts during ventricular diastole, slightly decreasing the abrupt rise in pressure and protecting the gill capillaries. Blood may be seen flowing via the sinus venosus, atrium, ventricle, and conus arteriosus of a fish embryo’s beating heart. Additionally, we can see several valves that stop blood from flowing backward. The sinoatrial valve is between the sinus venosus and the atrium. Between the atrium and the ventricle is where you’ll find the atrioventricular valve. The ventral aorta is a thin, somewhat short artery that soon splits into five pairs of afferent branchial arteries that supply the gills with deoxygenated blood. The first afferent artery supplies blood to the first two hemibranchs, whereas the second through fifth afferent arteries supply blood to both sets of holobranchs. The gill filaments’ multiple capillary beds are supplied by the arteries. The vessels that surround the gill pouches that these capillaries empty into are numerous. Each gill pouch has a pretrematic artery on the rostral side and a post-trematic artery on the caudal side. There are numerous cross-connections between these arteries. A collector loop is formed by each pair of pretrematic and post-trematic arteries, and it connects to an efferent branchial artery. Four pairs of efferent branchial arteries are used to carry oxygenated blood back to the heart from the gills. The dorsal aorta is made up of these four pairs that come together and curve posteriorly. The collecting loops and efferent arteries give rise to a variety of tiny arteries. The hyoidean epibranchial artery, a sizable artery that is connected to one of the paired dorsal aortae close to the spiracle to produce the internal carotid, emerges from the dorsal end of the first collector loop. As it enters the chondrocranium, the internal carotid advances and curves toward the midline, where it connects with the internal carotid from the opposing side. The two then split apart once more, becoming the major cerebral arteries. Where the internal carotid curves toward the midline, a stapedial artery splits off laterally. Some eye muscles receive blood from the stapedial artery.
The single median dorsal aorta is joined posteriorly to the anteriorly paired dorsal aortae. The caudal artery, which delivers blood into the tail, develops from this unpaired dorsal aorta’s later caudal extension. Along the route, the dorsal aorta sprouts several significant branches. Between the third and fourth efferent branchial arteries, the paired subclavian arteries first appear. The pectoral fins and the body wall musculature are both supplied with blood through the subclavians. Shortly posterior to the subclavians, the massive, unpaired celiac artery emerges. The coeliac artery divides into two branches: the pancreaticomesenteric artery, which sends branches to the stomach, pancreas, and intestine; and the gastrohepatic artery, which supplies the stomach and liver. The anterior mesenteric artery, the next significant branch of the dorsal aorta, feeds blood to the left side of the ileum. The pancreas, spleen, and stomach are all nourished by blood from the lienogastric artery. The posterior mesenteric artery, which feeds blood to the spiral valve and rectal gland, is the sole branch that extends backward the greatest. The renal arteries, which carry blood to the kidneys, and the iliac arteries, which supply the pelvic fins, are other pairs of paired arteries.
Systemic veins are the route via which blood returns to the heart. It might be more accurate to refer to some shark veins as sinuses. As the blood flows toward the heart, it is simpler to locate the veins that are going away from the heart. We can detect the massive, paired hepatic veins starting at the sinus venosus. They allow the hepatic portal system, which we shall study later, to drain blood. The common cardinal veins enter on either side of the sinus venosus. The four pairs of veins that get blood from the majority of the remainder of the body join at the common cardinal veins. Much of the blood from the caudal region of the body is drained via the massive posterior cardinal sinuses, one on each side of the body. They go into the veins of the cardinal points. Between the kidneys, on either side of the dorsal aorta, are a pair of posterior cardinal veins that take blood from the kidney veins. Blood travels from the posterior cardinal veins to the posterior cardinal sinus.
The Shark has two portal systems in addition to systemic veins: the renal portal system drains blood from the tail through the kidneys, and the hepatic portal system carries blood from the various regions of the digestive tract and spleen through the liver. Capillaries in the digestive system are the first part of the hepatic portal system. Several bigger veins that collect blood from various sections of the digestive tract feed the hepatic portal system with blood. Near the cranial tip of the pancreatic dorsal lobe, these veins converge. Blood from the stomach is carried by the gastric vein. The gastrointestinal vein follows the mesentery’s path of attachment to the pancreatic dorsal lobe, while the lienomesenteric vein receives blood from the rectal gland, spleen, pancreas, and left side of the intestine. The ventral lobe of the pancreas is where the pancreaticomesenteric vein originates. The hepatic portal vein, which enters the liver, is formed when these veins come together. The blood travels through several tiny sinusoids in the liver, where some nutrients are taken up and stored, the blood is filtered, and toxins are eliminated. The sinusoids in the liver unite to produce the hepatic veins, which pass through the sinus venosus. The caudal vein, which is protected by the vertebral hemal arch, is the first vein in the renal portal system. Along the dorsal surface of the kidneys, the caudal vein splits into two renal portal veins that deliver blood to the kidneys for filtering. Numerous tiny afferent renal veins carry blood into the kidneys, and numerous tiny efferent renal veins carry it out, returning it to the heart through the posterior cardinal veins.